The microcirculation of the renal medulla traps NaCI and urea deposited to the interstitium by the loops of Henle and collecting ducts. In the past, vasa recta have been conceptualized as simple "U-tube" diffusive exchangers. Discovery of aquaporin-1 (AQP1) water channels and the UTB urea carrier in descending vasarecta (DVR) endothelia revises our understanding. Solute gradients drive water loss from DVR to the interstitium across AQP1, a process that probably favors high medullary osmolality by shunting water to AVR and secondarily lowering blood flow to the inner medulla. In addition to AQP1, we have determined that a second mercurial insensitive route conducts small solute driven water flux across the DVR wall. The magnitude of transmural solute gradients responsible for driving water flux across the DVR wall is partially determined by DVR diffusive permeability. In turn, diffusive permeability to solute is both flow / shear rate dependent and subject to long term regulation. We propose the following aims: 1) We will test whether the water channel activity of UTB or other pathway(s) conducts the mercurial insensitive nonAQP1 water flux across DVR. We will measure water transport in UTB null DVR in which AQP1 has been disrupted or blocked by mercurials. We will measure the reflection coefficients of the non AQP1 pathway and determine whether, like UTB, water transport isphloretin sensitive. We will test whether nonAQP1 mediated water flux exhibits characteristics consistent with paracellular transport by examining its correlation with diffusive permeability and testing its bradykinin sensitivity. 2) We will test chronic effects on DVR transport in vehicle and vasopressin treated Brattleboro rats and rats subjected to chronic furosemide diuresis. We will test the effect of high angiotensin II states on permeability. 3) We will study the mechanisms responsible for flow dependence of DVR permeability. The effect of NO synthase blockade and NO donation will be tested. The ability of cyclic nucleotides to prevent flow dependent changes will be examined. 4) We will determine whether endothelial intracellular calcium elevations and NO generation accompanies changes in luminal flow / shear, and test whether superoxide generation modulates bioavailable NO. We will measure endothelial membrane potential and determine whether hyperpolarization occurs when DVR endothelia are subject to shear forces. [unreadable] [unreadable]